Paper of improved surface characteristics may be created by applying a thin layer of coating material to one or both sides of the paper. The coating is typically a mixture of a fine plate-like mineral, typically clay or particulate calcium carbonate; coloring agents, typically titanium dioxide for a white sheet; and a binder which may be of the organic type or of a synthetic composition. In addition, rosin, gelatins, glues, starches or waxes may be applied to paper for sizing.
Coated paper is typically used in magazines, commercial catalogs and advertising inserts in newspapers and in other applications requiring good quality color photo printing.
Coated ground-wood papers include the popular designation "lightweight coated" (LWC) paper. Ground-wood pulp contains lignin, which is a brown colored organic substance that acts as a binder in raw wood. The lignin reduces the cost of paper by increasing the yield of paper obtained from raw wood. At the same time it produces a paper with a surface that is less suitable for printing color photos and glossy images. Coating the ground-wood paper improves the printing quality while retaining the low cost advantage. For lightweight coated paper, coating weight is approximately thirty percent of total sheet weight. These grades of paper are popular with magazine publishers, direct marketers, and commercial printers as the lighter weight paper saves money on postage and other weight-related costs. With the increasing demand for lighter weight, lower cost coated papers, there is an increasing need for more efficiency in the production of these paper grades.
Paper is typically more productively produced by increasing the speed of formation of the paper, and coating costs are kept down by coating the paper while still on the papermaking machine. Because the paper is made at higher and higher speeds and because of the advantages of on-machine coating, the coaters in turn must run at higher speeds. The need in producing lightweight coatings to hold down the weight of the paper and the costs of the coating material encourages the use of short dwell coaters for their superior runnability at high machine speeds. Thus, high speed coating machines are key to producing lightweight coated papers cost-effectively.
Currently, coating applicators apply coating to the web in two separate manners. One is a direct application of a thin film by the coating applicator onto the moving web. The other is by application onto a transfer medium, i.e., an applicator roll, which then applies the thin film of fluid onto the web. Devices using either application approach may be classified as film applicators.
A typical film applicator has a coating application chamber that serves as an application region. One boundary wall of the application region is provided by the moving substrate, i.e. a paper web or blanket supported by a backing roll, an applicator roll, etc. Coating within the application chamber is effectively transferred onto the substrate. The substrate enters the application chamber through an overflow region where it meets the coating fluid at the dynamic contact line. The boundary layer adjacent to the moving substrate enters the application chamber containing air, and as it moves through the application chamber the air is replaced with coating. The substrate exits the application chamber at a metering element that controls the thickness of the applied coating. The application chamber provides the means for accelerating the coating fluid up to the speed of the moving substrate by allowing internal flow recirculation. The application chamber attenuates the cross-machine direction flow variations by permitting overflow through the baffle. In general, the residence time is short for the substrate, but can be relatively long for the coating fluid.
The major problem associated with this type of film applicator is the appearance of uncontrollable, nonuniform cross-machine direction and machine direction coat weight distributions on the substrate as the machine speed exceeds some critical speed limit. This speed limit depends upon the flow geometry in the application region and the rheological properties of the coating fluid. These non-uniformities exhibit a characteristic cross-machine length scale that appears to be proportional to the dimension characteristic of the active region where flow instabilities and disturbances take place.
Experimental data with a film applicator has revealed that the hydrodynamic instabilities induced by the presence of three-dimensional vortexes in the application chamber as well as flow disturbances created by the entrainment of air at the dynamic contact line and from the coating feed supply are important phenomena contributing to a nonuniform coat weight distribution. However, the relationship between these two phenomena is still unknown.
When a fluid is driven away from its stable equilibrium mode due to a change in operating conditions, it will often undergo a sequence of instabilities, each of which leads to a change in the spatial or temporal structure in the flow. In the present case, hydrodynamic instabilities develop as a result of the coating fluid undergoing transitions in different dynamic regimes, such as the shift from stable flow to an unstable flow as the Reynolds number (or machine speed) increases. The Reynolds number (Re) may be defined as: ##EQU1## Where .rho. is the density of the coating fluid, u is the characteristic velocity (substrate speed), L is the characteristic dimension of the active region where the state of flow undergoes different dynamic changes, and .mu. is the apparent viscosity of the coating fluid. The stability of flow in the active region can influence the uniformity of velocity and pressure profiles that, in turn, affect the coat weight distribution on the substrate.
Although air entrainment has been the subject of research in a number of areas related to a moving substrate entering into or contacting with an unpressurized liquid system, it is apparent that even at a low machine speed, there is still a lack of fundamental understanding of how air is entrained at the dynamic contact line, how much air volume enters with the moving substrate, and where the entrained air goes. Overall, any phenomenon observed at a low machine speeds tends to be magnified and become even worse as the machine speed increases.
For the case of flow in a pressurized film applicator, the amount of air being entrained increases as the machine speed increases. At the same time, this same speed increase and the increased volume of air create flow disturbances in the coating application chamber, disrupting the uniformity of the velocity and pressure profiles as well as the desired boundary layer adjacent to the moving substrate. At lower machine speeds, most of the air is successfully displaced or removed via the overflow region. At faster machine speeds, however, an increasingly larger volume of air is forced out through the overflow or possibly underneath the metering blade. This combined action of flow instability and uncontrolled air removal results in the emergence of the coat weight variations on the substrate.
What is needed is a film applicator that provides greater control over the conditions in the coating application chamber so that coating conditions can be adjusted as machine speed increases to overcome flow instability in the coating application chamber.